Method, apparatus and computer program product for implementing scan-chain-specific control signals as part of a scan chain

ABSTRACT

A method, apparatus and computer program product are provided for implementing scan-chain-specific control signals as an integral part of a scan chain. A scan input vector including scan data input and a scan control signal is applied to a register latch that forms the scan chain. The register latch includes a logic gate for combining a global clock control (THOLD) signal and the scan control signal. The scan control signal is routed around the register latch and including in a scan output vector including scan data output. Chain-specific control signals are eliminated from a clock control signal distribution tree used with the scan chain of the invention.

FIELD OF THE INVENTION

The present invention relates generally to the data processing field,and more particularly, relates to a method, apparatus and computerprogram product for implementing scan-chain-specific control signals asan integral part of a scan chain.

DESCRIPTION OF THE RELATED ART

In some of today's complex integrated circuits clock control signalsoften are used to gate the clock signals for all or part of the chip.These signals are used to turn off the clocks to the entire chip or todifferent parts of the chip as may be needed for a variety of reasons,such as power savings, chip configuration, thermal management, and thelike. The global clock control signals can be called THOLD signals.

In some integrated circuit chips, different sections of the clockcontrol tree may be gated by unique clock gate signals. These signalsallow the clocks to be turned off/on for different portions of thedesign. A design requirement that may be imposed is that all of thestorage elements that are controlled by one of these gated sections ofthe clock control tree must be in the same scan chain.

For example, FIG. 1A illustrates a conventional scan chain includingchain-specific clock gate signals called scan_enable signals. As shown,the scan_enable (0)-(N−1) signals are used to allow certain parts of thechip to continue running while other parts of the chip are quiesced andpossibly even scanned. Each latch, register, and the like in aparticular scan chain must be connected to the chain-specific clockcontrol signal for that scan chain. Currently, substantially manual,iterative processes that are both time consuming and cumbersome arerequired to design scan chains as shown in FIG. 1A.

FIG. 1B illustrates a conventional level sensitive scan design (LSSD)latch of FIG. 1A including a logic control block (LCB) receiving theclock and the chain-specific clock control signal for the particularscan chain and a register (REG) receiving the chain n scan_in and datain and providing chain n scan_out and data out.

FIG. 1C illustrates scan-testing operation of a prior art LSSD latchchain arrangement. The same scan_enable signals that are used to gatedifferent portions of the clock control tree are used in a logicbuilt-in self-test (LBIST) engine to enable the corresponding scanchains. This design requirement forces a dependency between gatedsections of the clock control tree and the scan chains. While thisarrangement allows the logic in individual scan chains to be stopped andpossibly scanned independently of the other scan chains, it does causesome serious difficulties in the logic design and timing closureprocess.

Using this clock control method for logic scan chains, the leaf-nodes ofthe clock-control distribution tree are not all functionally equivalentand the clock control tree leaf-nodes are associated with specific scanchains. Serious drawbacks for this clock control method result,currently causing significant additional time and tool resource toassure proper operation including:

Latches cannot be moved freely between scan chains for scan chainbalancing. To move a latch to a new scan chain, the clock controlsignals also have to be changed using the leaf-nodes that correspond tothe new scan chain. As the chip floorplan evolves, latches are movedaround in the floorplan. The clock control distribution tree may bedesigned to not move around. When a latch has moved in the floorplan itwould be desirable to connect that latch to the closest leaf-node ofclock control tree, treating the clock control tree like the clock griditself. However, this is often not possible without also changing thescan chain for the latch, because the leaf-nodes are associated with aspecific scan chain. The scan-chain balancing that is normally done latein the design cycle cannot be uncoupled from the design of the clockcontrol distribution tree.

SUMMARY OF THE INVENTION

Principal aspects of the present invention are to provide a method,apparatus and computer program product for implementingscan-chain-specific control signals as an integral part of a scan chain.Other important aspects of the present invention are to provide suchmethod, apparatus and computer program product for implementingscan-chain-specific control signals as an integral part of a scan chainsubstantially without negative effect and that overcome many of thedisadvantages of prior art arrangements.

In brief, a method, apparatus and computer program product are providedfor implementing scan-chain-specific control signals as an integral partof a scan chain. A scan input vector including scan data input and ascan control signal is applied to a register latch that forms the scanchain. The register latch includes a logic gate for combining a globalclock control (THOLD) signal and the scan control signal. The scancontrol signal is routed around the register latch and including in ascan output vector including scan data output.

In accordance with features of the invention, chain-specific controlsignals are eliminated from a clock control signal distribution treeused with the scan chain of the invention.

In accordance with features of the invention, an AND gate receives thescan control signal of the scan input vector and the global clockcontrol (THOLD) signal. The scan input vector can include a plurality ofbits N, one bit for scan data input and N−1 bits for chain-specificcontrol signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects andadvantages may best be understood from the following detaileddescription of the preferred embodiments of the invention illustrated inthe drawings, wherein:

FIG. 1A is a schematic diagram of a prior art clock control treearrangement including a chain specific clock control signal for eachscan chain;

FIG. 1B is a block diagram of a prior art level sensitive scan design(LSSD) latch of FIG. 1;

FIG. 1C is a block diagram illustrating scan testing operation of aprior art LSSD latch chain arrangement;

FIG. 2 illustrates an exemplary register latch for implementingscan-chain-specific control signals as an integral part of a scan chainin accordance with the preferred embodiment;

FIG. 3 illustrates an exemplary scan chain apparatus for implementingmethods for handling scan-chain-specific control signals as an integralpart of a scan chain in accordance with the preferred embodiment;

FIG. 4 illustrates exemplary input logic forming a part of a local clockbuffer (LCB) for implementing scan-chain-specific control signals as anintegral part of a scan chain in accordance with the preferredembodiment;

FIG. 5 illustrates another exemplary register latch for implementingscan-chain-specific control signals as an integral part of a scan chainin accordance with the preferred embodiment;

FIGS. 6 and 7 are block diagram representations illustrating a computersystem and operating system for implementing scan chain designs havingscan-chain-specific control signals as an integral part of a scan chainin accordance with the preferred embodiment;

FIG. 8 is a flow chart illustrating exemplary steps for implementingsettings change for scan-chain-specific control signals in accordancewith the preferred embodiment; and

FIG. 9 is a block diagram illustrating a computer program product in 5accordance with the preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with features of the invention, design methods areprovided whereby the chain-specific signals, such as scan_enables, areremoved from the clock-control distribution tree, and thesechain-specific signals are distributed as part of the scan chain itself.Each scan chain includes a scan vector of 2 or more bits, instead of theconventional singular scan input signal applied to a register and scanoutput signal provided by the register in a scan chain. In the new scanvector, one of the bits handles the scan data and the other bit or bitsare used for the chain-specific control signals.

In accordance with features of the invention, the clock-control treedesign is effectively de-coupled from the scan-chain design. Thisresolves many problems described above of the conventional clock controlmethod for logic scan chains. This de-coupling also permits designers touse some general design tools such as tree construction tools for theclock-control signal tree, and scan chain balancing and optimizationtools for routing the scan chain.

In accordance with features of the invention, an advantage is that sincethe scan chain data and the corresponding scan chain controls aredesigned and routed as a vector, reducing work and reducing thepossibility of connecting the wrong controls to the wrong scan chain.

Having reference now to the drawings, in FIG. 2, there is shown anexemplary register latch generally designated by the reference character200 for implementing scan-chain-specific control signals as an integralpart of a scan chain in accordance with the preferred embodiment.Register latch 200 enables removing the chain-specific control signals,such as scan_enables from the clock control (THOLD) tree. Register latch200 routes the chain-specific control signals, such as scan_enables aspart of each scan chain itself.

As illustrated in FIG. 2, an input signal SCAN_IN VECTOR is a two-bitvector, one bit (0) used for the scan data itself, and the other bit (1)is used, for example the scan_enable associated with that chain. Itshould be understood that the input signal SCAN_IN VECTOR in accordancewith the invention is not limited to a two-bit vector. For example, theinput signal SCAN_IN VECTOR can be implemented with an N-bit vector,with one bit used for the scan data and N−1 bits used for chain-specificcontrol signals. It should be understood that including chain-specificsignals as part of the scan chain itself is not be limited toscan_enable signals. There may be other chain-specific signals, whichadvantageously are also routed as part of the scan chains.

Register latch 200 includes an AND gate 202 receiving a global clockcontrol THOLD signal and the chain-specific control signal or thescan_enable signal from a vector-splitter 204 receiving the 2-bitscan_in vector. Register latch 200 includes a local clock buffer (LCB)210 receiving a clock input CLOCK and the AND gated global clock-controlTHOLD signal by the chain-specific control signal at the output of ANDgate 202. Register latch 200 includes a register REG 212 receiving adata input DATA_IN and a scan data input SCAN_IN from thevector-splitter 204. The register REG 212 provides a data outputDATA_OUT and a scan data output SCAN_OUT. The chain-specific controlsignal or the scan_enable signal from the vector-splitter 204 is routedaround the register latch 200 to provide at a vector-joiner 214 atwo-bit output vector SCAN_OUT VECTOR.

FIG. 3 illustrates an exemplary scan chain apparatus generallydesignated by the reference character 300 for implementing methods forhandling scan-chain-specific control signals as an integral part of ascan chain in accordance with the preferred embodiment. Scan chainapparatus 300 includes a scan_enable register 302 applying thechain-specific control signal or the scan_enable signal to a scan chain304 and a scan chain 306. The scan chain 306 is shown in simplified formwithout test connections. A logic built-in self-test (LBIST) engine 308provides the scan data to the scan chain 304. As a result of removingthe chain-specific signals from the clock-control tree, a generic signaldistribution tree 310 is coupled to the scan chain 304. The LBIST engine308 receives the scan_enable signal from the scan chain 304, eliminatingthe need for the conventional connection between the LBIST engine andthe scan_enable register.

As a result of the LBIST engine 308 receiving the scan_enable signalfrom the scan chain 304, the LBIST engine 308 can identify exactly whichscan chains have been successfully enabled. In the current state of theart the LBIST engine knows which scan chains should be enabled but doesnot have any feedback from the scan chains themselves to indicate thatthey are truly enabled.

Simply adding the gate 202 to the input side of the LCB 210 couldincrease the setup time required for the incoming global clock controlsignal (THOLD). Several things could be done to reduce this additionalsetup time. One option would be to add another input on the existinggate in the LCB instead of adding a new gate, as illustrated in FIG. 4.

FIG. 4 illustrates exemplary input logic forming a part of a local clockbuffer (LCB) generally designated by the reference character 400 forimplementing scan-chain-specific control signals as an integral part ofa scan chain in accordance with the preferred embodiment. As shown inFIG. 4, a logic gate 402 or a pair of logic gate 404, 406 for enablingspecific scan chains can be built into the LCB 400 or register design.For example, the three-input AND gate 402 can replace an existingtwo-input AND gate in the LCB, with the additional input receiving achain-specific control signal, such as, the scan_enable signal.

FIG. 5 illustrates another exemplary register latch to reduce setup timefor implementing scan-chain-specific control signals as an integral partof a scan chain generally designated by the reference character 500 inaccordance with the preferred embodiment. Register latch 500 includes anAND gate 502 receiving a global clock control THOLD signal and thechain-specific control signal or the scan_enable signal from avector-splitter 504 receiving the 2-bit scan_in vector.

In register latch 500 another method is used to remove the additionalsetup time in the LCB 510 by providing an additional special THOLDstaging latch REGTC 508 that is moved from outside to inside theregister latch definition. The AND gated global clock-control THOLDsignal by the chain-specific control signal at the output of AND gate502 is applied to the special THOLD staging latch REGTC 508.

Register latch 500 includes a local clock buffer (LCB) 510 receiving aclock input CLOCK and the output of REGTC 508. Register latch 500includes a register REG 512 receiving a data input DATA_IN and a scandata input SCAN_IN from the vector-splitter 504. The register REG 512provides a data output DATA_OUT and a scan data output SCAN_OUT . Thechain-specific control signal or the scan_enable signal from thevector-splitter 504 is routed around the register latch 500 to provideat a vector-joiner 514 a two-bit output vector SCAN_OUT VECTOR.

Referring now to the drawings, in FIGS. 6 and 7 there is shown acomputer system generally designated by the reference character 600 forimplementing scan-chain-specific control signals as an integral part ofa scan chain in accordance with the preferred embodiment. Computersystem 600 includes a main processor 602 or central processor unit (CPU)602 coupled by a system bus 606 to a memory management unit (MMU) 608and system memory including a dynamic random access memory (DRAM) 610, anonvolatile random access memory (NVRAM) 612, and a flash memory 614. Amass storage interface 616 coupled to the system bus 606 and MMU 608connects a direct access storage device (DASD) 618 and a CD-ROM drive620 to the main processor 602. Computer system 600 includes a displayinterface 622 coupled to the system bus 606 and connected to a display624.

Computer system 600 is shown in simplified form sufficient forunderstanding the present invention. The illustrated computer system 600is not intended to imply architectural or functional limitations. Thepresent invention can be used with various hardware implementations andsystems and various other internal hardware devices, for example,multiple main processors.

As shown in FIG. 7, computer system 600 includes an operating system630, an electronic design program 632, a chain-specific scan chaindesign program 634 of the preferred embodiment, a user interface 636,and a design library including chain-specific registers 638 of thepreferred embodiment. The chain-specific register design library 638includes one or more register designs for implementingscan-chain-specific control signals as an integral part of a scan chainin accordance with the preferred embodiment. For example, chain-specificregister design library 638 includes one or more register designsincluding, such as the illustrated register latch 200, LCB 400, andregister latch 500.

Referring now to FIG. 8, there are shown exemplary steps forimplementing a settings change for scan-chain-specific control signalsin accordance with the preferred embodiment. The chain-specific clockcontrol signals, such as scan_enables, typically do not change at allduring normal chip operation. The chain-specific clock control signalsare constant signals during chip operation. The exemplary steps shown inFIG. 8 can change the scan_enable settings of the preferred embodiment.

First globally stopping the clocks is provided as indicated in a block800. Then the scan_enable input values of the preferred embodiment arechanged as indicated in a block 802. A delay is provided to enable thescan_enable input values to ripple through the scan chain as indicatedin a block 804. Then globally turning the clocks back on is provided asindicated in a block 806.

Referring now to FIG. 9, an article of manufacture or a computer programproduct 900 of the invention is illustrated. The computer programproduct 900 includes a recording medium 902, such as, a floppy disk, ahigh capacity read only memory in the form of an optically read compactdisk or CD-ROM, a tape, or a similar computer program product. Recordingmedium 902 stores program means 904, 906, 908, 910 on the medium 902 forcarrying out the methods for implementing designs withscan-chain-specific control signals as an integral part of a scan chainin accordance with the preferred embodiment in the system 600 of FIGS. 6and 7.

A sequence of program instructions or a logical assembly of one or moreinterrelated modules defined by the recorded program means 904, 906,908, 910, direct the computer system 600 for implementingscan-chain-specific control signals as an integral part of a scan chainof the preferred embodiment.

While the present invention has been described with reference to thedetails of the embodiments of the invention shown in the drawing, thesedetails are not intended to limit the scope of the invention as claimedin the appended claims.

1. A method for implementing scan-chain-specific control signals as anintegral part of a scan chain formed by a plurality of register latches,said method comprising the steps of: applying a scan data input bit anda chain-specific scan control signal bit to a register latch; providingthe register latch with a logic gate for combining a global clockcontrol signal and the chain-specific scan control signal bit; routingthe chain-specific scan control signal bit around the register latch forcombining a scan data output bit of the register latch and the scancontrol signal bit; and providing a clock control signal distributiontree used with the scan chain with said clock control signaldistribution tree being implemented independently of the chain-specificscan control signal bit.
 2. The method for implementingscan-chain-specific control signals as recited in claim 1 wherein saidglobal clock control signal includes a global clock signal.
 3. Themethod for implementing scan-chain-specific control signals as recitedin claim 1 includes eliminating chain-specific control signals from aclock control signal distribution tree used with the scan chain.
 4. Themethod for implementing scan-chain-specific control signals as recitedin claim 1 wherein the step of applying a scan data input bit and a scancontrol signal bit to the register latch includes providing a pluralityof bits N including one bit for said scan data input bit and N-1 bitsfor each of a plurality of the chain-specific scan control signal bit ofa plurality of chain-specific control signals.
 5. The method forimplementing scan-chain-specific control signals as recited in claim 1wherein the step of providing the register latch with said logic gateincludes providing an AND gate for receiving said chain-specific scanenable control signal bit and the global clock control signal.
 6. Themethod for implementing scan-chain-specific control signals as recitedin claim 4 includes providing a global clock control staging latchcoupled to an output of said AND gate.
 7. The method for implementingscan-chain-specific control signals as recited in claim 1 wherein thestep of applying a scan data input bit and a chain-specific scan controlsignal bit to the register latch includes providing a logic built-inself-test (LBIST) engine for providing the scan data input bit to thescan chain; and said LBIST engine for receiving said chain-specific scancontrol signal bit from the scan chain.